Although several possible functions for the 3' terminal polyadenylic acid (poly A) portion of messenger RNA (mRNA) have been suggested, the experimental evidence at this time favors a stabilization role for this homopolymeric segment. Thus, the amount of ribonuclease (RNase) mediated hydrolysis of the non-polyadenylated portion of mRNA is an inverse function of the length of the poly A segment. This novel finding is not restricted to a single enzyme. The activity of RNases from a number of sources (human spleen, human plasma, bovine pancreas, Enterobacter sp., and Citrobacter sp.) is inhibited by a 3' terminal segment of poly A. This inhibition is subject to a number of controls that regulate the amount of hydrolysis of the non-polyadenylated portion of the molecule. Small increases in ionic strength, for example, induce reversal of inhibition. Certain polyamines (spermidine and spermine) and analogs of these compounds were also effective in this respect. (The total ionic strength of the reaction mixture was not affected by the concentrations of the compounds added.) Polyamines also serve to protect the enzymes from thermal inactivation and to renature inactivated enzyme. Thus, the level of intact substrate is determined by a combination of factors: (1) length of the 3' terminal poly A portion, (2) ionic strength, and (3) polyamine concentration. BIBLIOGRAPHIC REFERENCES: Karpetsky, T.P., Hieter, P.A., Frank, J.J. and Levy, C.C.: Polyamines, Ribonucleases, and the Stability of RNA. Mol. and Cell. Biochem., in press, 1977. Karpetsky, T.P., Boguski, M. and Levy, C.C. Polyadenylic Acid and Ribonucleases. Subcell. Biochem., in press, 1977.